A secondary battery, the use of which has been rapidly increased in recent years, is manufactured by stacking or winding an electrode assembly including a cathode, an anode, and a separator disposed between the cathode and the anode, placing the electrode assembly in a battery case formed of a metal container or a laminate sheet, and injecting an electrolyte into the battery case or impregnating the electrode assembly with the electrolyte.
One of the principal problems to be solved in connection with such a secondary battery is to improve the safety of the secondary battery. For example, the secondary battery may explode due to high temperature and high pressure which may be induced in the secondary battery due to the abnormal operation of the secondary battery, such as an internal short circuit, overcharge exceeding allowable current and voltage, exposure to high temperature, dropping, or deformation caused by external impact. Particularly for a pouch-shaped secondary battery, sealing force of a battery case is low with the result that an electrolyte may leak from the battery case.
On the other hand, the pouch-shaped secondary battery, which is a battery using a soft multi-layer film as a sheathing member, has several advantages that cannot be provided by a cylindrical secondary battery or a prismatic secondary battery using a metal case.
Typical examples of the advantages obtained by the pouch-shaped secondary battery may include low manufacturing cost, light weight, high safety secured through rupture of the pouch-shaped secondary battery before excessive internal pressure is accumulated in the pouch-shaped secondary battery, and excellent heat dissipation efficiency.
In connection with this case, a general structure of a conventional pouch-shaped secondary battery is shown in a front see-through view of FIG. 1.
Referring to FIG. 1, a pouch-shaped secondary battery 100 includes an electrode assembly 10, pluralities of electrode tabs 30 and 32 extending from the electrode assembly 10, electrode leads 20 and 22 respectively welded to the electrode tabs 30 and 32, and a battery case 40 in which the electrode assembly 10 is mounted.
The electrode assembly 10 includes cathodes and anodes successively stacked while separators are disposed respectively between the cathodes and the anodes. The electrode tabs 30 and 32 extend from corresponding electrode plates of the electrode assembly 10. The electrode leads 20 and 22 are electrically connected to the electrode tabs 30 and 32 extending from the corresponding electrode plates of the electrode assembly 10, respectively, for example, by welding. The electrode leads 20 and 22 are partially exposed from the battery case 10. To upper and lower surfaces of the electrode leads 20 and 22 are partially attached insulative films 50 and 52 to improve sealability between the battery case 10 and the electrode leads 20 and 22 and, at the same time, to secure electrical insulation between the battery case 10 and the electrode leads 20 and 22.
Meanwhile, the pouch-shaped secondary battery is manufactured as follows. First, as shown in FIGS. 1 to 3, an electrode assembly 10 is placed in a pouch-shaped battery case 40, and then an outer edge sealed portion 60 of an electrode assembly receiving part 90 of the battery case 40 is pressed by a high-temperature sealing tool 70 such that inner sealant layers 66, which have a low melting point, are welded and bonded to each other. Subsequently, as shown in FIG. 4, an outer end 68 of the outer edge sealed portion 60 is cut by a cutter as indicated by reference numeral 80.
This manufacturing method provides the pouch-shaped secondary battery 100 with high sealing strength and excellent sealing efficiency. However, the sealing tool 70 has a flat sealing surface with the result that some of the welded inner sealant layers 66 are discharged to the opposite ends of the inner sealant layers 66 during pressing of the outer edge sealed portion 60.
As a result, as shown in FIGS. 4 and 5, the inner sealant layers 66 are discharged to the electrode assembly 10 to form a bead 67. The bead 67 may contact the electrode assembly 10 in the battery case, damaging the electrode assembly 10 or causing a defect of the electrode assembly 10 and thus lowering durability of the battery.
That is, the inner sealant layers 66 discharged in the direction opposite to the electrode assembly 10 are removed through the outer end film cutting process. However, the inner sealant layers 66 discharged to the electrode assembly 10 cannot be cut with the result that the inner sealant layers 66 may cause the secondary battery to be defective.
Therefore, there is a high necessity for a technology that is capable of fundamentally solving the above problem.